In recent years, a demand for further improving an impact resistance property in a high-strength plated steel sheet used for an automobile and the like has been increasing. As techniques regarding a high-strength plated steel sheet excellent in impact resistance property, techniques described in Patent Literature 1 to Patent Literature 11, for example, have been proposed.
Patent Literature 1 describes a high-strength hot-rolled steel sheet excellent in bending workability and anisotropy of toughness which contains, by mass %, C: 0.08 to 0.15%, Si: 0.3 to 1.5%, Mn: 1.5 to 2.5%, P: ≦0.01%, S: ≦0.01%, Al: 0.01 to 0.05%, Ti: 0.03 to 0.15%, N: ≦0.004%, B: 0.0003 to 0.001%, O: ≦0.005%, and a balance composed of Fe and inevitable impurities, and has a ratio between an energy absorption amount Eab-L in a Charpy test specified in JISZ2242 (energy absorption amount in Charpy test (conducted at −40° C.) of test piece in which longitudinal direction is L direction) and Eab-C (Eab-L/Eab-C (energy absorption amount in Charpy test (conducted at −40° C.) of test piece in which longitudinal direction is C direction)) of not less than 0.9 nor more than 1.3.
Further, Patent Literature 2 describes a technique of manufacturing a high-strength hot-rolled steel sheet with a strength of tensile strength of 980 MPa or more, having a steel composition containing C: 0.08 to 0.20%, Si: less than 0.2%, Mn: greater than 1.0% and equal to or less than 3.0%, N: 0.01% or less, V: greater than 0.1% and equal to or less than 0.5%, Ti: 0.05% or more and less than 0.25%, and Nb: 0.005 to 0.10%, having a steel structure in which an area ratio of ferrite is 60% or more, and an area ratio of martensite is 5% or less, having an average grain diameter of ferrite of 5 μm or less, having a cleanliness d of 0.05% or less, and having a total number density of inclusions and precipitates each having an average grain diameter of 5 μm or more of 300 pieces/mm2 or less.
Further, Patent Literature 3 describes a high-tensile steel product excellent in low temperature toughness and with small strength anisotropy, having an X-ray random intensity ratio of {110} plane at a position of ¼ thickness of a sheet thickness from a surface of a steel sheet of 1.2 to 4.0, and having an X-ray random intensity ratio of {211} plane at a position of ½ thickness of the sheet thickness from the surface of the steel sheet of 1.2 to 4.0.
Patent Literature 4 describes a high-strength hot-rolled steel sheet having a composition in which, by mass %, C is limited to be 0.05% or more and less than 0.20%, Mn is limited to be 0.5% or more and less than 1.5%, sol.Al is limited to be 0.002% or more and less than 0.05%, Si is limited to be less than 0.1%, Cr is limited to be less than 0.1%, Ti is limited to be 0.01% or less, Nb is limited to be less than 0.005%, V is limited to be 0.01% or less, N is limited to be less than 0.005%, and a balance is composed of Fe and impurities, having a structure at a position at which a depth from a surface of steel sheet is ¼ thickness of a sheet thickness from the surface containing a ferrite phase, as a main phase, and a martensite phase of 10 to 30% in a volume ratio, in which an average crystal grain diameter of the ferrite phase is 1.1 to 3.0 μm, and an average grain diameter of the martensite phase is 3.0 μm or less.
Further, Patent Literature 5 describes a manufacturing method of a high-tensile hot-rolled steel sheet having a microscopic structure in which a volume ratio of ferrite is 80% or more, and an average ferrite grain diameter is less than 10 μm, in which a steel containing C: 0.05 to 0.30 wt %, Si: 2.0 wt % or less, Mn: 1.0 to 2.5 wt %, and Al: 0.05 wt % or less, containing one or two of Ti: 0.05 to 0.3 wt %, and Nb: 0.10 wt % or less, and containing a balance composed of Fe and inevitable impurities, is heated to a temperature of 950 to 1100° C., reduction in which a rolling reduction per one time becomes 20% or more is then performed at least two times or more, hot rolling is performed so that a finishing temperature becomes an Ar3 transformation point or more, cooling is performed in a temperature range of the Ar3 transformation point to 750° C. at a rate of 20° C./sec or more, retention in a temperature range of less than 750° C. to 600° C. is subsequently performed for a period of time of 5 to 20 sec, cooling is then performed again at a rate of 20° C./sec or more until the temperature reaches 550° C. or less, and the resultant is coiled into a coil at a temperature of 550° C. or less.
Further, Patent Literature 6 describes a high-strength hot-rolled thin steel sheet excellent in workability, fatigue property and low temperature toughness containing, as main components, C=0.04 to 0.15 mass %, Si≧1.0 mass %, Mn≧1.0 mass %, Nb≧0.005 mass %, Al=0.005 to 0.10 mass %, S≦ 0.01 mass % and Fe, having a microstructure mainly formed of ferrite and martensite, in which a space factor of ferrite (VF) is greater than 50%, an average grain diameter of ferrite (dF) is equal to or less than 5 μm and an average grain diameter of martensite (dM) is equal to or less than 5 μm, and having, as properties, a tensile strength (TS) of greater than 590 MPa, a yield ratio (YR) of equal to or less than 70%, a strength-ductility balance (tensile strength×total elongation) of equal to or greater than 18000 (MPa·%), a hole expansion ratio (d/d0) of equal to or greater than 1.2, a fatigue ratio of equal to or greater than 0.40, and a fracture transition temperature of equal to or less than −40° C.
However, each of the techniques described in Patent Literature 1 to Patent Literature 6 is the technique regarding the hot-rolled steel sheet, and thus cannot be applied to a manufacturing method of a steel sheet including a cold-rolling step and an annealing step. As a manufacturing method of a steel sheet excellent in impact resistance property including a cold-rolling step and an annealing step, techniques described in Patent Literature 7 to Patent Literature 11 have been proposed.
Further, Patent Literature 7 describes a high-strength alloyed hot-dip galvanized steel sheet excellent in energy absorption property that uses a steel sheet, as a base material, having a component composition containing C: 0.05 to 0.20 mass %, Si: 0.3 to 1.5 mass %, Mn: 1.0 to 2.5 mass %, P: 0.1 mass % or less, and a balance composed of Fe and inevitable impurities, and having a microstructure containing one or two of martensite and retained austenite of 25 to 50 volume % in total, and a balance composed of ferrite and bainite, and alloyed hot-dip galvanizing is applied to both surfaces of the steel sheet.
Further, Patent Literature 8 describes a high-tensile hot-dip galvanized steel sheet having a chemical composition containing, by mass %, C: 0.035 to 0.150%, Si: 0.05 to 0.60%, Mn: 2.0 to 4.0%, P: 0.015% or less, S: less than 0.0015%, sol.Al: 0.8% or less, N: 0.0031 to 0.015%, O: 0.0030% or less, Ti: 0.005 to 0.130%, Nb: 0 to 0.130%, in which a total amount of Ti and Nb is 0.055% or more, and a balance composed of Fe and impurities, and having a metallic structure in which an average crystal grain diameter of ferrite is 5.0 μm or less and an average grain diameter of hard second phase is 5.0 μm or less.
Further, Patent Literature 9 describes a manufacturing method of a high-strength cold-rolled steel sheet excellent in impact resistance property and shape fixability in which a slab having a composition of C: 0.08 to 0.18 mass %, Si: 1.00 to 2.0 mass %, Mn: 1.5 to 3.0 mass %, P: 0.03 mass % or less, S: 0.005 mass % or less, and T.Al: 0.01 to 0.1 mass %, and having a segregation degree of Mn (=(Mn concentration at center portion of slab−base Mn concentration)/base Mn concentration) of 1.05 to 1.10 is hot-rolled, the resultant is further cold-rolled, the resultant is then heated for a retention time of 60 seconds or more in a two-phase region or a single-phase region at 750 to 870° C. in a continuous annealing line, cooling is then performed in a temperature region of 720 to 600° C. at an average cooling rate of 10° C./s or less, cooling is then performed until the temperature reaches 350 to 460° C. at an average cooling rate of 10° C./s or more, retention is performed for 30 seconds to 20 minutes, and cooling is then performed until the temperature reaches room temperature to obtain a five-phase structure of polygonal ferrite, acicular ferrite, bainite, retained austenite, and martensite.
Patent Literature 10 describes a cold-rolled steel sheet excellent in impact absorption property having a hyperfine grain structure containing C, Si, Mn, Ni, Ti, Nb, Al, P, S, and N, having a ferrite phase whose volume fraction is 75% or more in which an average crystal grain diameter of ferrite is 3.5 μm or less, and having a structure of balance, other than the ferrite phase, which practically becomes a steel structure of tempered martensite.
Patent Literature 11 describes a high-ductility-type high-tensile cold-rolled steel sheet excellent in surface property and impact absorbency containing, by mass percent, C: 0.06 to 0.25%, Si: 2.5% or less, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.03% or less, Al: 0.1 to 2.5%, Ti: 0.003 to 0.08%, N: 0.01% or less, and a balance composed of Fe and inevitable impurities, in which the Ti content satisfies a relation of (48/14)N≦Ti≦(48/14)N+(48/32)S+0.01, and having a structure after cold rolling-recrystallization annealing being a structure containing retained austenite of 5% or more by volume ratio.